Bmil is required for Hedgehog-pathway driven medulloblastoma expansion

Inappropriate Hedgehog (Hh) signaling underlies development of a subset of medulloblastomas, and tumors with elevated HH signaling activity express the stem cell self-renewal gene Bmil. To test whether Bmil is required for Hh-driven medulloblastoma development, we varied Bmil gene dosage in transgenic mice expressing an oncogenic Hh effector, SmoA1, driven by a GFAP promoter. While 100% of SmoAl; Bmil+/+ or SmoA1; Bmil+/- mice examined between postnatal day 14 (P14) and P26 had typical medulloblastomas (N=29), tumors were not detected in any of the SmoA1; Bmil-/- animals examined (N=6). Instead, small, ectopic collections of cells were present in the region of greatest tumor load in SmoA1 animals, suggesting that medulloblastomas were initiated but failed to undergo expansion into frank tumors. Cells within these Bmil-/- lesions expressed SmoA1 but were largely non-proliferative, in contrast to cells in Bmil+/+ tumors (6.2% versus 81.9% PCNA-positive, respectively). Ectopic cells were negative for the progenitor marker nestin, strongly GFAP positive, and highly apoptotic, relative to Bmil+/+ tumor cells (29.6% versus 6.3% TUNEL-positive). The alterations in proliferation and apoptosis in SmoA1; Bmil-/- ectopic cells are associated with reduced levels of Cyclin D1 and elevated expression of cyclin-dependent kinase inhibitor p19Arf, two inversely-regulated downstream targets of Bmil. These data provide the first demonstration that Bmil is required for spontaneous de novo development of a solid tumor, suggest a crucial role for Bmil-dependent, nestin-expressing progenitor cells in medulloblastoma expansion, and implicate Bmil as a key factor required for Hh pathway-driven tumorigenesis.